Preparation and structural characterization of salts of oxotetrachlorotechnetium(V)

The reduction of pertechnitate(VII) ion, TeO4-, in aqueous HCl by hypophosphorous acid yields a dark green solution, as has been previously reported. From this solution olive green or gray-green crystalline solids can be isolated by addition of large cations such as (n-C4H9)4N+ or (Ph3P)2N+. Analytical and infrared data indicate that the anion in these crystalline solids is TcOCl4-, the tetrachlorooxotechnetate(V) ion. For the compound [(Ph3P)2N][TcOCl4], 1, a complete x-ray crystallographic structure determination has been carried out to define fully the oxotechnetate anion. Compound 1 crystallizes in space group Pna21 (No. 33) with unit cell constants of a = 21.618 (5) A, b = 16.870 (5) A, c = 9.658 (3) A, and Z = 4. The formula unit is the crystallographic asymmetric unit, and no crystallographic symmetry is imposed on either cation or anion. The TcOCl4- ion is, to a first approximation, square pyramidal, as would be expected, but is appreciably distorted, mainly in the sense of having significantly different trans Cl-Tc-Cl angles, 139.2 (1) and 153.7 (1)0. The symmetry is C/sub 2v/ within the experimental errors, with a Tc==O distance of 1.610 (4) A and a mean Tc-Cl distance of 2.31 +- 0.01 A. The Ph3PNPPh3+ ion has dimensions closely resembling those previously found for it in other compounds. Both compounds show a strong infrared absorption at ca. 1015 to 1020 cm-1 which may be assigned to stretching of the short Tc==O bond. Our conclusive identification of these green crystalline products as TcOCl4- salts is compared with previous reports in some of which a similar formulation was proposed while in others a different anionic species, Tc2Cl82-, was postulated. 2 figures, 4 tables